Digital phase lock loops are used in a variety of applications, one of which is the field of data communications. Often included as part of a digital phase lock loop is a voltage controlled oscillator (VCO) which is used to track a reference frequency by outputting a signal of a variably controlled frequency. The voltage controlled oscillator often takes the form of a ring oscillator which comprises a serial chain of stages, each of which provides a delay increment. The individual stages may be inverting or noninverting, but the output of the serial chain must be an inversion of its input to insure oscillation.
The frequency of the output of the voltage controlled ring oscillator is varied by changing the delay provided by each stage in the chain. The delay of each stage may be controlled by either an analog or a digital control signal applied to an input to the stage. In the case of an analog control signal, noise present on the control signal tends to cause jitter in the output of the oscillator.
Digital voltage controlled ring oscillators apply digital control signals to the inputs of stages in the oscillator to reduce the effect of such noise. Such digital ring oscillators typically utilize inverters as delay elements configured in a single serial chain. By varying the voltage input to one or more of the inverters, the delay provided by the inverters changes, thereby varying the output frequency of the oscillator. The period of the oscillator output increases, and thus its frequency decreases, as more delay elements are activated in the serial chain of the ring oscillator. The effect that a particular delay element has on the output frequency of the oscillator decreases proportionately as the number of delay elements increases. Thus, the delay increment provided by a particular delay element in a serial chain may shortened while increasing the number of delay elements in the serial chain. Shortening the delay increment permits the voltage controlled oscillator to more precisely follow the reference frequency.
By adding delay elements to the chain in a conventional digital voltage controlled oscillator, however, the circuitry required to decode the digital control input signals to the oscillator becomes necessarily more complex. Moreover, the minimum delay increment attainable even with more complex decoding circuitry is limited by the delay increment provided by a particular delay element. These types of conventional voltage controlled oscillators normally suffer from the delay increment being too large to provide sufficient frequency resolution to precisely follow the reference frequency.
It is an object of the present invention, then, to provide a digital controlled voltage oscillator which provides a smaller delay increment than that attainable with conventional oscillators, without increasing the complexity of circuitry required to decode the control signal inputs to the delay elements. It is a further object of the invention to provide such a digital voltage controlled oscillator which insures a monotonic output by either increasing or decreasing its output frequency for each change in the status of the digital control signal inputs.